Oil migration occurs in confectionery products that have a nut oil-based centre covered by a chocolate layer including truffles, pralines or peanut butter cup.

The lipids in the chocolate layer and the oil-based centre migrate to the opposite domains over time due to the differences in their concentrations and this diffusion and counter-diffusion of lipids is accompanied by changes in the lipid phase and microstructure of the chocolate layer - a process that can affect consumer acceptability of products.

The diffusion and counter-diffusion of lipids, said the authors, whose findings were published in the Journal of Food Engineering, can cause a softening in the chocolate layer, as well as a hardening of the centre and redistribution of flavours and fat bloom.

They concluded that tempering significantly reduced the diffusion in the core and intermediate layers of the cocoa butter samples studied while the outer layer exhibited similar diffusion profile for both tempered and the untempered samples.

“This reinforces the importance of the coating thickness for controlling oil migration in composite confectionery products,” noted the authors.

The researchers noted that over 70 per cent of the counter-diffusion of cocoa butter from the samples to the peanut oil occurred during the first week of storage, and they concluded that counter-diffusion could be controlled by adding solid particulates.

Method

Since tempering determines the microstructure of chocolate layer, the volumetric changes occurring during oil migration could potentially be influenced by tempering, reported the researchers.

They stressed though that no data is available regarding volumetric and dimensional changes in the chocolate layer upon oil migration and consequently their goal was to study peanut oil migration in confectionery samples evaluating diffusion, counter-diffusion, lipid phase transitions and dimensional changes.

The authors said that cylindrical tempered or untempered samples, composed of cocoa butter with or without addition of cocoa powder, were suspended in peanut oil and stored at 23ºC.

Oil migration kinetics was monitored using a novel pipette technique while diffusion and counter-diffusion were measured using gas chromatography, they added.

Results

The researchers found that tempering significantly reduced the diffusion of peanut oil but did not influence the change in solid fat content or counter diffusion of cocoa butter.

They noted that counter-diffusion occurred mostly at the interface of the sample and the peanut oil, and was reduced by addition of cocoa powder. “The diffusion was enhanced upon addition of cocoa powder indicating that cocoa powder particles reduce the strength of the fat crystal network resulting in higher diffusion.

“From aproduct development viewpoint, this means that well-tempered high fat barrier coatings could reduce oil migration in composite confectionery products,” said the authors.

The researchers explained that owing to a densely packed and stronger crystal network, the tempered samples offered higher resistance to the ingress of peanut oil which results in lower peanut oil diffusion.

“Scanning electron micrographs of well-tempered dark chocolate revealed spatially uniform crystal structures with well defined inter crystalline connections. On the other hand, large crystal clumps, a loosely packed crystal network with higher inter-crystalline spaces were found in under tempered chocolate.

The authors added that as oil migrating from the filling acts as a solvent for solid cocoa butter crystals, the dissolution of solid cocoa butter from the outer layer of the samples in the surrounding peanut oil substantially contributed to counter-diffusion.

They found that counter-diffusion was not affected by tempering and this suggests that dissolution of solid fat at the interface was not influenced by crystal form or arrangement.

“Presence of solid particulates lowered the amount of solid fat crystals available for dissolution per unit interfacial area.

Hence, samples containing cocoa powder exhibited relatively lower counter-diffusion as compared to samples. Our results indicate that counter-diffusion is possibly an interfacial phenomenon and could be controlled by adding solid particulates,” they concluded.